The ever increasing rate of petroleum consumption coupled with the current price demands of the primary oil producing countries has created the necessity of offshore oil and gas well drilling and producing operations in water of ever increasing depth and a resulting marine platform positioning or station keeping problem of ever increasing complexity and difficulty. In this latter regard, it is known that offshore oil and gas well drilling and producing operations in relatively shallow water, up to a depth of 2000 feet, for example, may be performed from a fixed tower-like structure which rests directly on and is firmly anchored directly to the ocean floor. At greater depths, floating platforms must be employed for such operations. These platforms must somehow be retained in a relatively fixed position over the well bore being drilled into or the well head on the ocean floor.
A floating oil well drilling installation, for example, has a floating drilling platform and a drill string extending from the platform to the ocean floor which is driven in rotation from the platform to drill a well bore into the floor. A floating oil or gas producing installation has a riser extending from the floating platform to the well head on the ocean floor and pumping means on the platform for pumping oil upwardly through the riser. The drill string and casing or riser, as the case may be, is thus fixed at its lower end, while its upper end moves horizontally and vertically with the floating platform. Floating at the water surface as it does, the platform is subject to displacement from its optimum position over the well bore or well head by wind, wave, and/or ocean current action. Accordingly, platform positioning or station keeping means must be provided for retaining the platform in position against the action of such forces.
Two general types of platform positioning techniques, one passive and the other dynamic, have been devised for this purpose. The passive platform positioning technique involves simply anchoring or tethering the platform in a fixed position with cables extending from the platform to anchors fixed to the ocean floor. The dynamic platform positioning technique involves sensing departure of the floating platform from its optimum position and continuously driving the platform back toward such position. The present invention is concerned with a dynamic platform positioning technique.
A wide variety of dynamic platform positioning techniques have been devised. A few of these are described in the following patents:
______________________________________ 3,121,954 3,508,512 3,148,653 3,588,796 3,187,704 3,730,126 3,191,570 3,886,887 3,311,079 3,948,201 3,369,516 ______________________________________
The dynamic platform positioning techniques described in many of these patents involve sensing and generating signals representing, in terms of a selected coordinate system, the angle of a connecting line extending from the platform to a fixed reference position on the ocean floor, and controlling a platform propulsion system in response to these signals in such a way as to maintain the platform in a desired horizontal position relative to the reference. The connecting line is a cable in U.S. Pat. No. 3,187,704, and a casing or drill string in U.S. Pat. No. 3,191,570. U.S. Pat. No. 3,148,653 shows two connecting lines, one a cable and one a drill string.
The prior art dynamic floating platform positioning systems which utilize such a connecting line suffer from two major deficiencies which this invention overcomes. The first deficiency resides in the fact that these platform positioning systems assume the connecting line to be essentially linear along its full length from the floating platform to the ocean and rely on an angle measurement of one end only of the connecting line. In some cases, this angle measurement is made at the floating platform as in U.S. Pat. No. 3,121,954 and in other cases, at the ocean floor as in U.S. Pat. No. 3,191,570. U.S. Pat. No. 3,148,653, which employs two separate connecting lines, i.e., a cable and a drill string, makes the cable angle measurement at the platform and the drill string angle measurement at the ocean floor.
In actual practice, however, the connecting line becomes increasingly more nonlinear and its nonlinearity becomes more time variable as the water depth increases due to several factors, such as surface wave induced platform motion which produces traveling stress waves or undulations in the connecting line, ocean currents, the weight of the connecting line itself, changing connecting line tension and the like.
The second deficiency is related to the phase lag which exists between true platform position and sensor measured platform position, particularly at depths exceeding approximately 2500 feet, when the bottom angle of the connecting line is used for position determination. This phase lag is due to the transit time required for platform motion induced stress waves or undulations to propagate downwardly through the connecting line to the bottom angle sensor. When a position measuring system which introduces such phase lag is incorporated into a closed loop dynamic positioning system, the stability of the closed loop system is degraded, and in some instances, may become completely unstable.
As a consequence of the foregoing and other deficiencies, the existing dynamic floating platform positioning systems of the character described are useful only in relatively shallow water up to depths on the order of 2500 feet. On the other hand, present and near future offshore oil operations contemplate accurate marine platform positioning in water up to depths of 6000 feet and more. Accordingly, there is a need for an improved dynamic marine platform positioning or station keeping system.